From 3GPP release R99 onwards service applications in a telecom network can request active location determination from the mobile terminal locating system in the Public Land Mobile Network (PLMN). Active location retrieval (ALR) is specified in CAMEL-Phase 3 as “current location retrieval”. Details of ALR are given in 3GPP TS 23.078 and 3GPP TS 29.002. ALR is an enhancement to Any Time Interrogation (ATI) that was introduced in CAMEL Phase 1. ATI is part of the Mobile Application Part (MAP) protocol for communication between entities in the PLMN. MAP_ATI allows entities to get from the Home Location Register (HLR) specific details of a mobile subscriber such as subscriber location, subscriber state, mobile terminal registration number etc. The type of information requested is provided in the “RequestedInfo” field, see FIG. 6 showing a compilation of the MAP-ATI elements as presented in 3GPP TS 29.002. ALR is in fact requesting the option “currentLocation”.
The flow of messages and involved entities is outlined in FIG. 1. The HLR receives the MAP_ATI from a requestor and detects the request for current location so it treats it further as an ALR. The mobile station (MS) for which the ALR is intended is identified by either IMSI or MSISDN. Where as the HLR is in the Home PLMN (H-PLMN), the mobile station is connected in a visited PLMN (V-PLMN) handled by the visited MSC (VMSC). The V-PLMN is the PLMN where the mobile station is currently connected and registered as being present. When a mobile station is in the H-PLMN, then the V-PLMN for that mobile station coincides at that moment with the H-PLMN for that mobile station. It shall be noted that the requestor does not need to be in the H-PLMN. The HLR acts on the ALR with sending a MAP-PSI (Provide subscriber info) towards the VMSC. MAP-PSI has the same RequestedInfo field as MAP-ATI in which the HLR copies the received “RequestedInfo” field from the MAP-ATI. The VMSC acts by paging the mobile station to obtain the current location.
Paging is to be understood as a variety of possibilities to obtain the actual position of the Mobile station. The mobile station can report the co-ordinates it has measured with its own (built-in) GPS, by means of enhanced triangulation (e.g. Difference in Time of Arrival, DToA), Radio direction finding system etc. Commonly the combination of the mobile station and network related equipment is addressed as locating system which is controlled by the VMSC. Inherent to all position measurements is that it requires radio air capacity as well as Radio access network (RAN) resources to perform and/or report measurements. For a more detailed view on methods for determination of a position of a mobile station, related measurements and calculations reference should be made to 3GPP TS25.305.
The paging result comprises the measured location reported directly to the VMSC by the mobile station or reported by a network based function. The VMSC forwards this information to the HLR with a MAP_PSI response message and the HLR forwards to the requestor in a MAP_ATI response. ALR offers in this way a good possibility to telecom services to obtain accurate position information of mobile stations. Problems however arise with the increasing amount of services that make use of position information. A few examples;                Location based charging; prior to connecting, the services checks the position of both mobiles, and if they are in the same area a reduced charging rate is used comparable to making a local call on the fixed telephone network.        Home zone charging; which reduces the charging rate, when you call from or being called in an area that is considered to be around your living address.        Find closest buddy. This is a service of the family and friends group which locates the closest mobile party on your personal Family & Friends list.        Exit/arrival SMS. This service monitors the location of a mobile and issues an SMS when a mobile station enters or leaves a defined area.        
Another aspect is that services can be chained, meaning that find closest buddy is followed by local based charging and then by a prepaid service. Each of the services may perform a location request on their own.
The growing number of current location requests put a growing demand on already scarce radio air spectrum capacity and radio access network resources. Operators facing a growing demand for more air bandwidth due to more data traffic, mobile internet and mobile TV look for alternative methods to lower the impact of location requests on their radio air spectrum capacity and radio access network resources.